1. Field of the Invention
The field of this invention relates to a process for reducing the sulfur content of coal.
2. Prior Art
The problem of air pollution due to the emission of sulfur oxides when sulfur-containing fuels are burned has received increasing attention in recent years. It is now widely recognized that sulfur oxides can be particularly harmful pollutants since they can combine with moisture to form corrosive acidic compositions which can be harmful and/or toxic to living organisms in very low concentrations.
Coal is an important fuel, and large amounts are burned in thermal generating plants primarily for conversion into electrical energy. One of the principal drawbacks in the use of coal as a fuel is that many coals contain amounts of sulfur which generate unacceptable amounts of sulfur oxides on burning. For example, coal combustion is by far the largest single source of sulfur dioxide pollution in the United States at present, and currently accounts for 60 to 65% of the total sulfur oxide emissions.
The sulfur content of coal, nearly all of which is emitted as sulfur oxides during combustion, is present in essentially two forms: inorganic, primarily metal pyrites, and organic sulfur. The inorganic sulfur compounds are mainly iron pyrites, with lesser amounts of other metal pyrites and metal sulfates. The organic sulfur may be in the form of thiols, disulfide, sulfides and thiophenes (substituted, terminal and sandwiched forms) chemically associated with the coal structure itself. Depending on the particular coal, the sulfur content can be primarily in the form of either inorganic sulfur or organic sulfur. Distribution between the two forms varies widely among various coals.
In the United States, except for Western coals, the bulk of the coal produced is known to be high in pyrite. Both Appalachian and Eastern interior coals have been analyzed to be rich in pyritic and organic sulfur. Generally, the pyritic sulfur represents from about 25% to 70% of the total sulfur content in these coals.
Heretofore, it was recognized that it would be highly desirable to remove (or at least lower) the sulfur content of coal prior to combustion. A number of processes, for example, have been suggested for removing the inorganic (pyritic) sulfur from coal.
For example, it is known that at least some pyritic sulfur can be physically removed from coal by grinding the coal, and subjecting the ground coal to froth flotation or washing processes. While such processes can desirably remove some pyritic sulfur, these processes are not fully satisfactory because a significant portion of the pyritic sulfur is not removed. Attempts to increase the portion of pyritic sulfur removed have not been successful because these processes are not sufficiently selective. Because the process is not sufficiently selective, a large portion of coal can be discarded along with ash and pyrite.
There have also been suggestions heretofore to chemically remove sulfur from coal. For example, U.S. Pat. No. 3,768,988 to Meyers, issued Oct. 30, 1973, discloses a process for reducing the pyritic sulfur content of coal involving exposing coal particles to a solution of ferric chloride. The patent suggests that in this process ferric chloride reacts with pyritic sulfur to provide free sulfur according to the following reaction process: EQU 2FeCl.sub.3 +FeS.sub.2 .fwdarw.3FeCl.sub.2 +S
while this process is of interest, a disadvantage of this process is that the liberated sulfur solids must then be separated from the coal solids. Processes involving froth flotation, vaporization and solvent extraction are proposed to separate the sulfur solids. All of these proposals, however, inherently represent a second discrete process step with its attendant problems and cost which must be employed to remove the sulfur from coal.
In another approach, U.S. Pat. No. 3,824,084 to Dillon issued July 16, 1974, discloses a process involving grinding coal containing pyritic sulfur in the presence of water to form a slurry, and then heating the slurry under pressure in the presence of oxygen. The patent discloses that under these conditions the pyritic sulfur (for example, FeS.sub.2) can react to form ferrous sulfate and sulfuric acid which can further react to form ferric sulfate. The patent discloses that typical reaction equations for the process at the conditions specified are as follows: EQU FeS.sub.2 H.sub.2 C+7/2O.sub.2 .fwdarw.FeSO.sub.4 +H.sub.2 SO.sub.4 EQU 2feSO.sub.4 +H.sub.2 SO.sub.4 +1/2O.sub.2 .fwdarw.Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O
these reaction equations indicate that in this particular process the pyritic sulfur content continues to be associated with the iron as sulfate. While it apparently does not always occur, a disadvantage of this is that insoluble material, basic ferric sulfate, can be formed. When this occurs, a discrete separate separation procedure must be employed to remove this solid material from the coal solids to adequately reduce sulfur content. Several other factors detract from the desirability of this process. The oxidation of sulfur in the process does not proceed at a rapid rate, thereby limiting output for a given processing capacity. In addition, the oxidation process is not highly selective such that considerable amounts of coal itself can be oxidized. This is undesirable, of course, since the amount of coal recovered from the process is decreased.
Numerous other methods have been proposed for reducing the sulfur content of coal. For example, U.S. Pat. No. 3,938,966, to Kindig et al issued Feb. 17, 1976, discloses treating coal with iron carbonyl to enhance the magnetic susceptibility of iron pyrites to permit removal with magnets. In summary, while the problem of reducing the sulfur content of coal has received much attention, there still exists a present need for a practical method to more effectively reduce the sulfur content of coal.